The present invention relates to the selective hydrogenation of aliphatic nitriles such as adiponitrile to omega aminonitriles such as .epsilon.-aminocapronitrile, employing only ammonia with a catalyst containing finely dispersed rhodium on basic metal oxide supports.
Rhodium has found practical applications in hydrogenations. Rhodium metal itself, and inorganic rhodium oxides and salts supported on an inert support have been used in certain applications. Examples of processes employing such inert rhodium materials are contained in P. N. Rylander, Catalytic Hydrogenation in Organic Synthesis (N.Y. 1979).
U.S. Pat. No. 4,389,348 (S. E. Diamond et al.) discloses that rhodium-based catalysts selectively hydrogenate dinitriles to aminonitriles in the presence of an aprotic solvent for the dinitrile and ammonia. However, in Example 10 of the U.S. Patent, increased amounts of ammonia even in the presence of aprotic solvent such as THF completely prevented rhodiumcatalyzed hydrogenation of the dinitrile.
Y. Takagi et al., Scientific Papers Institute Physical & Chemical Research (Japan), Vol. 61, No. 3, pp. 114-17 (1967) discloses processes for hydrogenating nitriles with unsupported rhodium catalysts. In particular, rhodium hydroxide prepared by adding various amounts of sodium hydroxide to a hot aqueous solution of rhodium chloride was used as a catalyst for the hydrogenation of adiponitrile. Table II of the reference indicates that increased amounts of sodium hydroxide as a further additive increased the yield of hexamethylenediamine, but eventually retard the reaction rate. Lithium hydroxide was a superior additive. The yields were in most cases lower than the 80% yield reported using rhodium oxide prepared by the fusion of rhodium chloride with sodium nitrate.
M. Freifelder et al., J. Am Chem. Soc., vol. 82, pp. 2386-2389 (1960) employed 5% rhodium on alumina to hydrogenate aliphatic nitriles and especially 3-indoleacetonitrile. Ammonia was present to suppress production of secondary amines, but also reduced the catalytic activity of the rhodium. Ammonia is known to suppress secondary amine formation and reduce activity with other Group VIII metal catalysts.
U.S. Pat. Nos. 2,208,598 and 2,257,814, each to Rigby (Dupont 1940), and German Pat. Nos. 836,938 (1952), 848,654 (1952) and 954,416 (1956), all to BASF, disclose various catalytic processes directed to producing omegaaminonitriles from dinitriles employing miscellaneous catalysts including Raney nickel and iron, but not specifically any of the platinum group. See also U.S. Pat. No. 2,762,835 to Swerdloff (Celanese 1956).
Italian Pat. No. 845,999 to Montecatini Edison S.p.A. (1969) discloses the hydrogenation of dinitriles having one or two carbons shorter than adiponitrile (e.g. succinonitrile) in the presence of a rhodium catalyst and ammonia to produce an omega-aminonitrile.
None of these patents provides a process with high selectivity to aminonitrile (compared to by-products, e.g., diamine and secondary amine) at high conversions of dinitrile with high rates of catalyst turnover and long catalyst life in the presence of ammonia as the only solvent.